The technologies concerning the development and manufacturing of semiconductors have advanced exponentially. Nowadays, the processed transistors have diminished in size and area of an integrated circuit (IC). This is an extremely important fact since a greater range of devices and integrated circuits can be employed and implemented, providing the possibility of adding more functions to a single chip, thus reducing the required area and power consumption.
Reference voltage circuits are essential blocks for the development of analog and mixed signal blocks such as voltage regulators, AD converters, flash memories, DRAM memories, switched capacitor circuits, RF communication blocks, comparators, etc. The power required by advanced systems has achieved increasingly low and tight levels, particularly in RF and biomedical system applications due to the limitations in the available power. In this case, the reference voltage circuits must be able to operate at wide supply voltage ranges, from a few volts through below 1 volt due to the broad operation range of the RF systems, where the operation voltage depends directly on the incoming RF signal power at the antenna. Therefore, to provide a sub-1V reference voltage with extremely low power consumption to keep up with this natural evolution, new topologies are required to support the growing and dynamic RF market demand.
Conventional circuits used as temperature-compensated reference sources are usually built by adding two terms easily found in IC design. The first one is the forward bias voltage of a PN junction (Vf), while the second one is the thermal voltage (Vt) equal to KT/q, where K is the Boltzmann constant, T is absolute temperature, and q is the electron charge. Note these two terms have opposite behaviors over temperature namely CTAT (Complementary-to-Absolute Temperature, or with a negative temperature coefficient), and PTAT (Proportional-to-Absolute Temperature, or with a positive temperature coefficient) respectively. When designed and properly scaled, the addition of these two terms provides a first order temperature-compensated reference voltage usually in the order of 1.2V in silicon (Si) technologies, also known as bandgap voltage.
However, the reference voltages have changed the concept of circuits because of the evolution of the technologies (lower supply voltages) and the need for extremely reduced energy consumption. Thus, in a search for new solutions capable of overcoming the challenges posed by these new technologies, several methods to develop reference voltages below 1 volt have been developed. Among the first proposed approaches to develop reference voltages below 1 volt is the one presented in 1999 by H. Banba., S. Hitoshi, U. Akira, and M; Takeshi. They presented a reference voltage derived from the addition of two currents generated by a single feedback loop, such currents being proportional to a forward biased diode voltage and a thermal voltage (KT/q). The reference voltage obtained depends on the relation of two resistors. The authors claim a 518 mV+/−15 mV reference voltage for a 3 sigma spread. The minimum simulated supply voltage was 0.84V and the reported current consumption was 2.2 uA. These values do not satisfy the need to operate with a power consumption below 1 uA.
In a second solution, S. Mehmarmanesh, M. B. Vahidfar, H. A. Aslanzadeh, and M. Atarodi propose a reference voltage based on a structure in a regulated current mode with some feedback loops to achieve operation at low voltage and low power. The authors claim that the technique employed features a high impedance power supply. On the other hand, K. N. Leung, and P. Mok propose a reference voltage based on the thermal coefficient difference presented by the threshold voltage (Vth) of a PMOS transistor and a NMOS transistor. Thus, a reference voltage can be built to achieve low temperature variation mutually compensating the transistors' Vth temperature variations, that diminish with temperature on a linear basis in a first-order approximation (i.e., the Vth of a MOS transistor has a CTAT behavior). The minimum supply voltage achieved was 1.4V; the minimum consumption achieved was 9.7 uA, and the temperature variation was 36.9 parts per million per Celsius degrees (ppm/° C.). The presented values do not satisfy the need to operate with a power consumption below 1 uA and a low supply voltage.
Giuseppe de Vita and Giuseppe Iannaccone proposed a reference voltage of extremely low power consumption operating at a supply voltage between 0.9V and 4V. The proposed structure features a 70 nA current consumption. The authors claim that the reference voltage generator features a temperature variation of 10 ppm/° C., that is achieved by means of a perfect elimination of the mobility dependence on temperature, compensation for the channel modulation effect, and the absence of the body effect. A. Aldokhaiel, A. Yamazaki, and M. Ismail proposed a reference voltage that uses the so-called body-drive technique, that allows the circuit to be operated at low supply voltages without requiring low-threshold voltage devices. The authors claim that the technique used features a greater common-mode range. S. Mingoo, D. Sylvester, D. Blaauw, S. Hanson, and G. Chen have recently invented what they call an improved reference generator, that consists of two serially connected transistors operating in the weak inversion region and with different threshold voltages where the transistor with larger Vth is connected as a diode while the transistor with lower Vth is serially connected to ground. The lower Vth transistor consists of a native transistor whose gate is connected to ground. The authors claim that any combination of devices will work providing there is a considerable Vth difference. The reference voltage is obtained at the intermediate node between the two transistors and its value depends on the size of both transistors and the bias point of the transistor with larger Vth. The expression for the reference voltage contains two terms with opposite temperature dependence differentiating the Vth's and the (KT/q) thermal potential [7].
It is important to mention that a large part of the previously mentioned studies satisfy the need to implement a generator with a reference voltage below 1V. However, the power consumption in most of them is in the order of microamperes, evidencing a greater energy consumption required for the systems to operate perfectly.
Based on the aforementioned studies, improvements and new technologies are being developed and marketed in order to provide new viable solutions with high performance and low power consumption. The U.S. Pat. No. 8,058,863 proposes a bandgap reference voltage circuit encompassing MOS transistors connected to bipolar transistors, apart from employing operational amplifiers. A CTAT voltage is scaled down by a threshold voltage of a NMOS transistor, and a reference voltage lower than or equal to 1V is provided by resistances respectively connected to the bipolar transistors. The patent in question evidences the fact that an additional resistance must be adjusted so that the reference voltage is independent of the temperature. Thus, the inconvenience of using resistances, operational amplifiers, and bipolar transistors is clear since bipolar transistors usually require a large bias current causing high power consumption. The need of a greater number of devices to build the reference voltage circuit is also evident.
Likewise, the US patents and patent applications US20090096509, US20080136504, U.S. Pat. No. 7,259,543, US20060108994 and U.S. Pat. No. 6,501,256 provide solutions to generate sub-iv reference voltages requiring the use of operating amplifiers, bipolar transistors, diodes and/or resistors which make it difficult to achieve low power consumption and low voltage operation simultaneously. Furthermore, these circuits do not feature low-complexity and reduced-size solutions, thus evidencing the high costs involved.
Also according to Huang et al in “A CMOS sub-1V nanopower current and voltage reference with leakage compensation”, a voltage and current reference was developed based on a modified polarized SCM structure using 3 bias currents. The reference voltage is below 1V. However, its effective use for supplies lower than 1V is doubtful. The proposed circuit features an apparently simple concept and good compatibility with several MOS manufacturing processes. The temperature compensation depends on the injection of a leakage current making the solution not so reliable.
In “A simple subthreshold CMOS voltage reference circuit with channel length modulation compensation”, Huang et al proposed reference voltage generators working at a low power supply voltage but requiring many resistors for good behavior making it unable to achieve low power consumption. Furthermore, in “A new voltage reference topology based on subthreshold MOSFETs”, published in ESSCIRC 2002, a similar solution like the previous one is proposed but with a minimum supply voltage of 1.2V, i.e., out of the sub-1V operation range.
Doyle et al propose in “A CMOS sub-bandgap reference circuit with 1V power supply voltage”, the use of bipolar transistors, resistors and operational amplifiers to obtain a reference voltage below 1V. Such circuits are relatively complex, occupy a large area, and do not consume low power.
It is also important to consider the US patent application US 20100327842, that proposes a reference voltage generator composed basically of a first transistor with a given threshold voltage and operating in weak inversion, and a second transistor connected in series with the first one. The second transistor also operates in a weak inversion and has a larger threshold voltage than the first transistor. The gate electrode of the second transistor is electrically coupled to the drain electrode of the second transistor, forming an output for the reference voltage. Furthermore, this patent shows several configurations of voltage reference circuits with different bias voltages and various circuit topologies seeking reference voltages depending on temperature.
However, even though the solution proposed by US 20100327842 attains low voltage operation, consumes low power, and consists of a small and simple two-transistor circuit operating at weak inversion, there is a clear part-to-part variation in the reference voltage, thus compromising its temperature compensation. Furthermore, in order to obtain a certain driving capability, an output buffer is required substantially increasing the power consumption. Finally, the architecture is extremely dependant on the power supply voltage as it defines the transistors' inversion level. The reported area in this work is 1 mm2, an area quite considerable.